I hope, if I'm not saying this right the mods will correct me . . .
It seems vacuuming is for the purpose of getting rid of visible debris. So it could help to vacuum to waste and not have the algae take up residence in the filter. If you don't see visible debris/dead algae, it probably does no benefit to vacuum to waste. Brushing is still very important to make the chlorine more effective in killing the algae by disrupting it's biofilm (did I say that right?)
So it would seem - vacuum visible algae first then brush sides, walls, look for niches that could hide algae, stairs. The solar panel and hoses if not used could build up algae. Is that possible?
Also it's difficult to keep the FC up at shock level. You know the "shock level and maintain" TFP mantra.
If the chlorine is getting used up quickly, its harder to "maintain" shock level.
How often are you able to check FC levels? That would use up the regents a lot quicker but necessary to keep the chlorine levels up. Are you running low on regents?
I'm not sure what to recommend re: DE. I've only used it a few times but I've read it can help but it will make the psi rise quicker and when you backwash, it all washes away and you have to add more. That's the extent of my experience.
I'll let the mods take on that question.